I had the unmitigated pleasure of re reading the novel this past weekend. I had completely forgotten this family and their role in bringing Michael home. The book says they were not the sharpest knives in the drawer, but because of their honor they were the family to turn to when it became necessary to mediate disputes, and to be used when "hostages" were held during negotiations. I imagine if anyone offended the honor of this family all hell would rain down.

I do not think they were the "sixth" family in New York, however.
That said I am now convinced there were six families because throughout the book Puzo talks about the Corleones doing battle with the Five families.